1. Technical Field
The present invention relates to a scale having an inductively scannable graduation, a method for producing this scale, and a position measuring instrument having this scale.
2. Background Information
Position measuring instruments that operate on the inductive measurement principle have a scale which has an inductively scannable graduation. The graduation includes a succession of electrically conductive graduation elements spaced apart from one another. In the measurement mode, the graduation is scanned by a scanning unit, which has at least one excitation winding and one scanning winding. These windings are preferably applied two-dimensionally on a circuit board. An excitation current impressed on the excitation winding generates a chronologically alternating electromagnetic excitation field, which is varied as a function of position by the array of graduation elements, and, as a result, a position-dependent scanning signal is generated in the scanning winding.
Inductively scannable scales and position measuring instruments that function inductively have the advantage that they are relatively insensitive to being contaminated. In particular, they are insensitive to liquids, such as water and oils, in the space between the scale and the scanning unit, and they are therefore especially suitable for measuring angles and lengths in machine tools.
In European patent disclosure EP 0 743 508 A2, an inductively scannable scale and an inductive position measuring instrument are described. It is explained that the graduation elements include a material with high electrical conductivity and are mounted on a circuit board material, such as FR4. Because of its electrical insulation, the circuit board material is especially suitable as a carrier for the graduation elements. Because of the mechanical disadvantages of the circuit board material, it is proposed in EP 0 743 508 A2 that the graduation elements be mounted directly on a steel substrate or an invar substrate, or, in other words, on an electrically conductive material, to improve the mechanical stability.
As a condition for the use of a metal carrier, EP 0 743 508 A2 says that the electrical conductivity of the material of the carrier merely needs to be very much lower than the electrical conductivity of the material of the graduation elements. In practice, however, it has been found that when conventional steel is used as the carrier, the scanning signals are relatively weak, which is why in practice, only inductively scannable scales with an insulating carrier have become established. In such scannable scales, the graduation elements are mounted on an electrically insulating carrier, in particular on circuit board material. However, it is difficult to produce a scale of this kind in great lengths, and because of the circuit board material it is not resistant to ambient factors.